1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexing (OFDM) system, and more particularly to a method for transmitting data using Vertical-Bell Labs Layered Space-Time (V-BLAST) coding in a multiple-input multiple-output (MIMO) OFDM communication system.
2. Description of the Related Art
Various transmission and reception schemes in a wireless mobile communication system have been proposed to achieve high-quality and high-capacity multimedia data transmission within the limited frequency resources available. Demands for a method that is effective at removing the fading phenomenon occurring in mobile wireless channels are gradually increasing in order to achieve the high-speed transmission of such multimedia data. Extensive research is being conducted in relation to MIMO technology, which uses multiple transmission/reception antennas to be applied to the fourth generation (4G) mobile communication system for very high-speed multimedia data transmission. The MIMO technology is a well-known technology, so a detailed description thereof will be omitted.
The MIMO technology includes a V-BLAST technique developed by Bell Laboratories of Lucent Technologies, USA. According to the V-BLAST technique, a complex coding in a transmission side is not required, and different signals are transmitted from each transmission antenna. Therefore, it is possible to greatly increase the data transmission rate by the V-BLAST technique.
In addition, according to the V-BLAST technique, data streams are independently encoded and are transmitted from different antennas. A reception side performs an ordered successive interference cancellation (OSIC) procedure to remove any interference between signals transmitted from the different transmission antennas. An OFDM technique using multiple carriers to increase the frequency efficiency and to efficiently remove multipath fading may be employed in the V-BLAST system.
The V-BLAST scheme uses an open loop technique, which does not need any information feedback from a reception side to a transmission side. Some schemes to feedback necessary information from a reception side to a reception side for performance improvement have been proposed. For instance, an Adaptive Bit and Power Allocation (ABPA) scheme is a representative example. According to the ABPA, a reception side of the MIMO system using a V-BLST detector on the basis of a two-dimensional water pouring principle determines various modulation schemes (i.e. the number of bits and power to be allocated) for each of sub-channels according to a channel state, and feedbacks the determined modulation scheme to a transmission side.
The ABPA scheme provides an optimum performance with respect to the bit error rate (BER). However, the ABPA scheme is disadvantageous in that the feedback information from the reception side to the transmission side is excessively required and thus a large number of operations is required for bit and power allocation. Therefore, in an actual operating environment considering feedback delay, etc., the ABPA scheme may exhibit much lower performance than expected in theory.
In order to compensate for the problems of the ABPA scheme, a Simplified Bit Allocation (SBA) scheme for allocating an equal number of bits to partial sub-channels having an excellent channel characteristic has been proposed to reduce the amount of operation required to allocate bits and the amount of feedback information from a reception side to a transmission side. Particularly, according to the SBA scheme, an equal modulation scheme is applied only to selected sub-channels, and no bits are allocated to unselected sub-channels.
The SBA scheme also has a problem in that an overall transmission rate is reduced due to the sub-channels to which no bits have not been allocated. In order to prevent such a problem in the SBA scheme, a higher-level modulation technique than the original one must be used for the selected sub-channels. When the higher-level modulation technique is applied to selected sub-channels as described above, there is a problem in that the probability of symbol error on the selected channel increases.
Since a typical V-BLAST detection technique as described above performs a hard decision in an interference removing procedure, it has a problem in that the gain of a channel code for performing the soft channel decoding decreases. Since an input value to a channel decoder in an OSIC procedure is a hard decision value as described above, there is a problem in that a channel code gain of a soft decoder decrease such as that of a zigzag decoder.
As described above, when information can be feedback from a reception side to a transmission side in a MIMO-OFDM system, various schemes for improving the performance of a typical V-BLAST algorithm may be considered.
For example, as described above, the ABPA scheme may be considered as representative. According to the ABPA scheme, a reception side determines various modulation schemes (i.e. the number of bits and power to be allocated) for each sub-channel according to a channel state on the basis of a two-dimensional water pouring principle, and feedbacks to a transmission side the determined modulation scheme. The ABPA scheme performs optimally in view of the bit error rate performance, but has difficulty in finding an optimum V-BLAST detection sequence because each sub-channel has a different signal constellation. This is because such a V-BLAST detection sequence becomes ineffective when signal constellations used with antennas differ. According to the ABPA scheme, as the number of transmission antennas increases, the amount of operation for bit and power allocation and feedback information from a reception side to a transmission side increases more rapidly.
As a scheme for reducing the number of operations for bit allocation and feedback information from a reception side to a transmission side, the Simplified Bit Allocation (SBA) scheme has been proposed as described above. According to the SBA scheme, a modulation scheme having an equal signal constellation is applied to each sub-channel by allocating an equal number of bits to the sub-channels, thereby significantly reducing the amount of operation for finding an optimum detection sequence and a sub-channel for bit allocation. According to the SBA scheme, an equal number of bits is allocated to each sub-channel, and the transmission side does not need information for power allocation, so that feedback information from the reception side to the transmission side is reduced. However, since the SBA scheme applies an equal modulation scheme only to selected sub-channels but allocates no bit to unselected channels, an overall transmission rate is reduced due to the channels to which no bit has been allocated. In order to prevent such a problem in the SBA scheme, a higher-level modulation technique than the original one must be used for the selected sub-channels. As a result, the SBA scheme has advantages in that it increases the probability of symbol error on sub-channels to which bits have been allocated and it has a much inferior bit error rate performance to the ABPA scheme.